@Philip123
A guest can leave at any time and either cancel or not.
If they cancel then your cancellation policy comes into effect.
Sounds like a longer term stay? Did you get an email that sounded like you were doing the guest a favour by cancelling his stay - if so and you agreed then you have pretty much been scammed by AirBnB. It's such a sneaky way they have of getting you to agree to a cancellation and then making it your fault.
On the other hand - there could be fraud involved. Had they run out of credit. Or were there extenuating circumstances? All you can do is contact AirBnB and ask what the situation is.